The invention relates to apparatus and process for obtaining samples of drilling cuttings from active mud systems, specifically that which permits a representative sample to be collected and subjected to continuous quantitative analysis.
During the drilling of a well, mud is circulated downhole to carry away drill cuttings. The cuttings are a view into the characteristics of the drilled strata. In an active mud system, the mud is circulated in a loop; pumped from the mud tank, downhole to the drilling bit, up the annulus to the surface, and back to the mud tank for separation of cuttings, and separation of fine solids in tanks, reconstitution of mud ingredients and reuse. Conventionally, in the first step of separation, before returning to the mud tank, the cuttings are passed over an inclined shaker for separating the largest cuttings from the mud which falls through screens to a tank therebelow. The cuttings are sampled and discarded in a sump. The sampling of the cuttings enables the driller to review the strata being drilled.
The cuttings obtained at the surface must be associated with the strata being drilled. Cuttings cannot be directly related to the actual position of the drilling bit due to the lag associated with the return of the mud from the bit to the surface. This association is obtained using a variety of techniques, the simplest being to correlate the flow rate of mud, the volume of the well bore, mud circulation system and the bit position. Other methods which assist in minimizing inherent inaccuracies with cross-strata blending and the like include matching downhole gamma ray emissions with that measured from cuttings.
Simply, the objective is to obtain samples for analyzing the cuttings in a sequential manner, indexed to the drilling.
Conventionally, some of the cuttings are sampled in some manner or another. Analyses include batch storage of sample in collection tubes, removed manually and analyzed after the fact. Alternatively or in combination, cuttings are stored in small cotton sample bags for storage or later analysis.
One long-time applied method of capturing cuttings includes directing cuttings from the discharge of the shaker and over a plate. The plate has a plurality of holes in it and has converging side walls which funnel the cuttings across the plate. Some of the cuttings pass through the holes and fall into a bucket under the plate. While the intent is to obtain a representative sample, the slip stream approach and stratification of the flow over the plate results in a sample that is less than representative of the entire cuttings population.
Further, the resulting sample, collected in a bucket provides the means for merely an overall qualitative analysis, not a discrete quantitative analysis relative to indexed depths within the wellbore. Accurate assessment of the formation strata is not possible with large, indiscrete sampling. As the sample buckets are only emptied periodically, they may fill to overflowing allowing valuable sample to be lost. The introduction of fluids, such as heavy rains or waves in offshore drilling may cause cuttings to be washed out of the sample collecting bucket resulting in the irretrievable loss of geological data.
Dissatisfaction with errors arising from the simple past methods has caused others to attempt more comprehensive systems of sampling an U.S. Pat. No. 5,571,962 to Georgi et al., it is recognized that certain errors in associating collected cuttings to the strata being drilled. Accordingly, Georgi suggest measuring gamma ray emissions in collected cuttings and comparing them with well drilling logs of same. Georgi provides a continuous sample collection apparatus in which cutting samples are routed by an auxiliary mud pump through a flow line to a mini-shaker connected to an inclined shale shaker. The mini-shaker has assemblies of varying mesh sizes so as to separate the cutting based on particle size. The cuttings are subsequently washed with fluid to remove fine particles which flow out of the top of a settling pipe. The larger, more dense cuttings settle to the bottom by gravity and are collected in transparent storage vessels which permit qualitative inspection, examination for gamma ray emissions and for employing ultraviolet fluorescent techniques. Georgi anticipates further automating the process using mechanical carousels to rotate the collection vessels and means to collect simultaneous duplicate samplings for future analysis.
Like the older sloped and perforated plate technique, the screens of Georgi""s mini-shaker may not provide a well mixed sample, having performed a further stage of a slip-stream screening separation, risking segregation of the sample including loss of sample and plugging; and while it has been suggested to automate the removal of storage vessels, there is not disclosed apparatus for doing same or for determining when they should be changed out or how to associate them with the drilling.
Other sample collectors, such as that described in U.S. Pat. No. 4,718,289 to Barrett collect only and do not send sample for analysis. In this case of buckets the sample collection devices must be removed from the stream of cuttings by the operator and are therefore very subjective with regards to sampling frequency.
U.S. Pat. No. 4,287,761 to Moffat et al. describes a flow chamber which allows for the continuous analysis of drilling mud using visual scrutiny through sight glasses and a hydrocarbon sensor mounted in the continuous flow chamber. Discrete samples are not retained for any future analysis.
Method and sample catcher apparatus are provided for obtaining a representative sample of cuttings, and for selecting representative sub-samples for storage and analysis. In one embodiment, the apparatus comprises an inclined sampling screw conveyor which intercepts the entire drill cuttings flow from a shaker and a vaned metering rotor which accepts a fixed volume of sub-sample from the discharge of the sampling screw conveyor, wherein the rate of extraction is less than the continuous stream of drill cuttings, and directs it to one of, or both of, a series of analytical instruments or a indexed carousel of sample bags. The rate of penetration (ROP) and the recirculation rate of mud is determined so as to establish a lag ROP, being the ROP as it was at the time the drill cuttings were drilled. The carousel is controlled to index advance for properly associating the bag contents with the drilling. The apparatus further comprises an analytical conveyor for directing at least a portion of the sample stream past one or more analytical devices. The conveyor is transparent to the particular emission characteristics of the instrument. Further, it is preferred that the analyzed sub-sub-sample is discharged from the analytical screw into a transparent cylinder for visual and qualitative analysis. A carousel can also be provided for associating the transparent cylinder contents with the drilling.
The apparatus enables a novel process for obtaining and analyzing cuttings from a shale shaker comprising receiving the whole shaker discharge of cuttings, conveying and mixing the whole cuttings in a sampling screw conveyor and discharging the bulk to waste. A plurality of discrete, metered and representative volume samples of cuttings are extracted from adjacent the discharge of the sampling conveyor with the vaned metering rotor. Discrete samples are obtained at a rate which is proportional to the lag ROP. The sample is discharged as a substantially continuous sample stream or as sub-sample streams to the analytical instruments or into a series of sample bags which are successively filled, advanced and replaced in a manner dictated by the rate of drilling. The sampling conveyor is advanced sufficiently quickly to ensure the whole flow is accommodated, yet slow enough to properly mix a representative sample. The advance rate of the sampling conveyor can also be linked to Lag ROP, as is the metering rotor, and the sample bag carousel. The sample bags are further tracked and identified according to the drilling depth, associated by drilling rate and mud lag.
More preferably, the process for analyzing cuttings is further enhanced by selecting a portion of the sample stream and directing it past a series of quantitative analyses which are conducted through the emission transparent wall of the analytical conveyor, such analyses including gamma emission rate, nuclear density, laser reflection spectrometry, fluorescence and sonic testing. The present method permits the sample to be analyzed prior to discharging to the sample bags. Even more preferably is to direct the sample, once passed by the analytical instrument into a vertical transparent container for further visual and qualitative analysis.